Auto merge of rust-lang#118134 - Nilstrieb:rollup-kyo1l6e, r=Nilstrieb

Rollup of 6 pull requests Successful merges: - rust-lang#116085 (rustdoc-search: add support for traits and associated types) - rust-lang#117522 (Remove `--check-cfg` checking of command line `--cfg` args) - rust-lang#118029 (Expand Miri's BorTag GC to a Provenance GC) - rust-lang#118035 (Fix early param lifetimes in generic_const_exprs) - rust-lang#118083 (Remove i686-apple-darwin cross-testing) - rust-lang#118091 (Remove now deprecated target x86_64-sun-solaris.) r? `@ghost` `@rustbot` modify labels: rollup
rust-lang-ci · Nov 21, 2023 · 0ff8610 · 0ff8610
2 parents e24e5af + fa8878b
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diff --git a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
@@ -35,7 +35,7 @@ use crate::session_diagnostics::{
     LifetimeReturnCategoryErr, RequireStaticErr, VarHereDenote,
 };
 
-use super::{OutlivesSuggestionBuilder, RegionName};
+use super::{OutlivesSuggestionBuilder, RegionName, RegionNameSource};
 use crate::region_infer::{BlameConstraint, ExtraConstraintInfo};
 use crate::{
     nll::ConstraintDescription,
@@ -763,7 +763,14 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         let err = LifetimeOutliveErr { span: *span };
         let mut diag = self.infcx.tcx.sess.create_err(err);
 
-        let fr_name = self.give_region_a_name(*fr).unwrap();
+        // In certain scenarios, such as the one described in issue #118021,
+        // we might encounter a lifetime that cannot be named.
+        // These situations are bound to result in errors.
+        // To prevent an immediate ICE, we opt to create a dummy name instead.
+        let fr_name = self.give_region_a_name(*fr).unwrap_or(RegionName {
+            name: kw::UnderscoreLifetime,
+            source: RegionNameSource::Static,
+        });
         fr_name.highlight_region_name(&mut diag);
         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
         outlived_fr_name.highlight_region_name(&mut diag);

diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -107,6 +107,14 @@ impl<K: Hash + Eq, V> interpret::AllocMap<K, V> for FxIndexMap<K, V> {
         FxIndexMap::contains_key(self, k)
     }
 
+    #[inline(always)]
+    fn contains_key_ref<Q: ?Sized + Hash + Eq>(&self, k: &Q) -> bool
+    where
+        K: Borrow<Q>,
+    {
+        FxIndexMap::contains_key(self, k)
+    }
+
     #[inline(always)]
     fn insert(&mut self, k: K, v: V) -> Option<V> {
         FxIndexMap::insert(self, k, v)

diff --git a/compiler/rustc_const_eval/src/interpret/machine.rs b/compiler/rustc_const_eval/src/interpret/machine.rs
@@ -49,6 +49,14 @@ pub trait AllocMap<K: Hash + Eq, V> {
     where
         K: Borrow<Q>;
 
+    /// Callers should prefer [`AllocMap::contains_key`] when it is possible to call because it may
+    /// be more efficient. This function exists for callers that only have a shared reference
+    /// (which might make it slightly less efficient than `contains_key`, e.g. if
+    /// the data is stored inside a `RefCell`).
+    fn contains_key_ref<Q: ?Sized + Hash + Eq>(&self, k: &Q) -> bool
+    where
+        K: Borrow<Q>;
+
     /// Inserts a new entry into the map.
     fn insert(&mut self, k: K, v: V) -> Option<V>;
 

diff --git a/compiler/rustc_const_eval/src/interpret/memory.rs b/compiler/rustc_const_eval/src/interpret/memory.rs
@@ -692,6 +692,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         Ok((&mut alloc.extra, machine))
     }
 
+    /// Check whether an allocation is live. This is faster than calling
+    /// [`InterpCx::get_alloc_info`] if all you need to check is whether the kind is
+    /// [`AllocKind::Dead`] because it doesn't have to look up the type and layout of statics.
+    pub fn is_alloc_live(&self, id: AllocId) -> bool {
+        self.tcx.try_get_global_alloc(id).is_some()
+            || self.memory.alloc_map.contains_key_ref(&id)
+            || self.memory.extra_fn_ptr_map.contains_key(&id)
+    }
+
     /// Obtain the size and alignment of an allocation, even if that allocation has
     /// been deallocated.
     pub fn get_alloc_info(&self, id: AllocId) -> (Size, Align, AllocKind) {

diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
@@ -128,12 +128,6 @@ lint_builtin_type_alias_generic_bounds = bounds on generic parameters are not en
 lint_builtin_type_alias_where_clause = where clauses are not enforced in type aliases
     .suggestion = the clause will not be checked when the type alias is used, and should be removed
 
-lint_builtin_unexpected_cli_config_name = unexpected `{$name}` as condition name
-    .help = was set with `--cfg` but isn't in the `--check-cfg` expected names
-
-lint_builtin_unexpected_cli_config_value = unexpected condition value `{$value}` for condition name `{$name}`
-    .help = was set with `--cfg` but isn't in the `--check-cfg` expected values
-
 lint_builtin_unpermitted_type_init_label = this code causes undefined behavior when executed
 lint_builtin_unpermitted_type_init_label_suggestion = help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
 

diff --git a/compiler/rustc_lint/src/builtin.rs b/compiler/rustc_lint/src/builtin.rs
@@ -33,7 +33,6 @@ use crate::{
         BuiltinMutablesTransmutes, BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns,
         BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasGenericBounds,
         BuiltinTypeAliasGenericBoundsSuggestion, BuiltinTypeAliasWhereClause,
-        BuiltinUnexpectedCliConfigName, BuiltinUnexpectedCliConfigValue,
         BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit,
         BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub, BuiltinUnsafe,
         BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
@@ -60,7 +59,6 @@ use rustc_middle::ty::GenericArgKind;
 use rustc_middle::ty::ToPredicate;
 use rustc_middle::ty::TypeVisitableExt;
 use rustc_middle::ty::{self, Ty, TyCtxt, VariantDef};
-use rustc_session::config::ExpectedValues;
 use rustc_session::lint::{BuiltinLintDiagnostics, FutureIncompatibilityReason};
 use rustc_span::edition::Edition;
 use rustc_span::source_map::Spanned;
@@ -2889,26 +2887,3 @@ impl EarlyLintPass for SpecialModuleName {
         }
     }
 }
-
-pub use rustc_session::lint::builtin::UNEXPECTED_CFGS;
-
-declare_lint_pass!(UnexpectedCfgs => [UNEXPECTED_CFGS]);
-
-impl EarlyLintPass for UnexpectedCfgs {
-    fn check_crate(&mut self, cx: &EarlyContext<'_>, _: &ast::Crate) {
-        let cfg = &cx.sess().parse_sess.config;
-        let check_cfg = &cx.sess().parse_sess.check_config;
-        for &(name, value) in cfg {
-            match check_cfg.expecteds.get(&name) {
-                Some(ExpectedValues::Some(values)) if !values.contains(&value) => {
-                    let value = value.unwrap_or(kw::Empty);
-                    cx.emit_lint(UNEXPECTED_CFGS, BuiltinUnexpectedCliConfigValue { name, value });
-                }
-                None if check_cfg.exhaustive_names => {
-                    cx.emit_lint(UNEXPECTED_CFGS, BuiltinUnexpectedCliConfigName { name });
-                }
-                _ => { /* expected */ }
-            }
-        }
-    }
-}
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
@@ -179,7 +179,6 @@ early_lint_methods!(
             IncompleteInternalFeatures: IncompleteInternalFeatures,
             RedundantSemicolons: RedundantSemicolons,
             UnusedDocComment: UnusedDocComment,
-            UnexpectedCfgs: UnexpectedCfgs,
         ]
     ]
 );

diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
@@ -553,21 +553,6 @@ pub enum BuiltinSpecialModuleNameUsed {
     Main,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(lint_builtin_unexpected_cli_config_name)]
-#[help]
-pub struct BuiltinUnexpectedCliConfigName {
-    pub name: Symbol,
-}
-
-#[derive(LintDiagnostic)]
-#[diag(lint_builtin_unexpected_cli_config_value)]
-#[help]
-pub struct BuiltinUnexpectedCliConfigValue {
-    pub name: Symbol,
-    pub value: Symbol,
-}
-
 // deref_into_dyn_supertrait.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_supertrait_as_deref_target)]

diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
@@ -3439,6 +3439,7 @@ declare_lint_pass! {
         UNCONDITIONAL_PANIC,
         UNCONDITIONAL_RECURSION,
         UNDEFINED_NAKED_FUNCTION_ABI,
+        UNEXPECTED_CFGS,
         UNFULFILLED_LINT_EXPECTATIONS,
         UNINHABITED_STATIC,
         UNKNOWN_CRATE_TYPES,

diff --git a/compiler/rustc_middle/src/mir/interpret/mod.rs b/compiler/rustc_middle/src/mir/interpret/mod.rs
@@ -525,13 +525,6 @@ impl<'tcx> TyCtxt<'tcx> {
         self.alloc_map.lock().reserve()
     }
 
-    /// Miri's provenance GC needs to see all live allocations. The interpreter manages most
-    /// allocations but some are managed by [`TyCtxt`] and without this method the interpreter
-    /// doesn't know their [`AllocId`]s are in use.
-    pub fn iter_allocs<F: FnMut(AllocId)>(self, func: F) {
-        self.alloc_map.lock().alloc_map.keys().copied().for_each(func)
-    }
-
     /// Reserves a new ID *if* this allocation has not been dedup-reserved before.
     /// Should only be used for "symbolic" allocations (function pointers, vtables, statics), we
     /// don't want to dedup IDs for "real" memory!

diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
@@ -1587,7 +1587,6 @@ supported_targets! {
     ("armv7r-none-eabihf", armv7r_none_eabihf),
 
     ("x86_64-pc-solaris", x86_64_pc_solaris),
-    ("x86_64-sun-solaris", x86_64_sun_solaris),
     ("sparcv9-sun-solaris", sparcv9_sun_solaris),
 
     ("x86_64-unknown-illumos", x86_64_unknown_illumos),

diff --git a/compiler/rustc_target/src/spec/targets/x86_64_sun_solaris.rs b/compiler/rustc_target/src/spec/targets/x86_64_sun_solaris.rs
diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
@@ -49,9 +49,9 @@ const MIR_OPT_BLESS_TARGET_MAPPING: &[(&str, &str)] = &[
     ("i686-unknown-linux-musl", "x86_64-unknown-linux-musl"),
     ("i686-pc-windows-msvc", "x86_64-pc-windows-msvc"),
     ("i686-pc-windows-gnu", "x86_64-pc-windows-gnu"),
-    ("i686-apple-darwin", "x86_64-apple-darwin"),
     // ARM Macs don't have a corresponding 32-bit target that they can (easily)
     // build for, so there is no entry for "aarch64-apple-darwin" here.
+    // Likewise, i686 for macOS is no longer possible to build.
 ];
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]

diff --git a/src/ci/docker/host-x86_64/dist-various-2/Dockerfile b/src/ci/docker/host-x86_64/dist-various-2/Dockerfile
@@ -48,9 +48,6 @@ ENV \
     AR_x86_64_pc_solaris=x86_64-pc-solaris2.10-ar \
     CC_x86_64_pc_solaris=x86_64-pc-solaris2.10-gcc \
     CXX_x86_64_pc_solaris=x86_64-pc-solaris2.10-g++ \
-    AR_x86_64_sun_solaris=x86_64-sun-solaris2.10-ar \
-    CC_x86_64_sun_solaris=x86_64-sun-solaris2.10-gcc \
-    CXX_x86_64_sun_solaris=x86_64-sun-solaris2.10-g++ \
     CC_armv7_unknown_linux_gnueabi=arm-linux-gnueabi-gcc-9 \
     CXX_armv7_unknown_linux_gnueabi=arm-linux-gnueabi-g++-9 \
     AR_x86_64_fortanix_unknown_sgx=ar \
@@ -84,8 +81,6 @@ COPY host-x86_64/dist-various-2/build-fuchsia-toolchain.sh /tmp/
 RUN /tmp/build-fuchsia-toolchain.sh
 COPY host-x86_64/dist-various-2/build-solaris-toolchain.sh /tmp/
 RUN /tmp/build-solaris-toolchain.sh x86_64  amd64   solaris-i386  pc
-# Build deprecated target 'x86_64-sun-solaris2.10' until removed
-RUN /tmp/build-solaris-toolchain.sh x86_64  amd64   solaris-i386  sun
 RUN /tmp/build-solaris-toolchain.sh sparcv9 sparcv9 solaris-sparc sun
 COPY host-x86_64/dist-various-2/build-x86_64-fortanix-unknown-sgx-toolchain.sh /tmp/
 RUN /tmp/build-x86_64-fortanix-unknown-sgx-toolchain.sh
@@ -120,7 +115,6 @@ ENV TARGETS=$TARGETS,wasm32-wasi
 ENV TARGETS=$TARGETS,wasm32-wasi-preview1-threads
 ENV TARGETS=$TARGETS,sparcv9-sun-solaris
 ENV TARGETS=$TARGETS,x86_64-pc-solaris
-ENV TARGETS=$TARGETS,x86_64-sun-solaris
 ENV TARGETS=$TARGETS,x86_64-unknown-linux-gnux32
 ENV TARGETS=$TARGETS,x86_64-fortanix-unknown-sgx
 ENV TARGETS=$TARGETS,nvptx64-nvidia-cuda

diff --git a/src/ci/docker/host-x86_64/x86_64-gnu-tools/checktools.sh b/src/ci/docker/host-x86_64/x86_64-gnu-tools/checktools.sh
@@ -25,7 +25,16 @@ cat /tmp/toolstate/toolstates.json
 python3 "$X_PY" test --stage 2 check-tools
 python3 "$X_PY" test --stage 2 src/tools/clippy
 python3 "$X_PY" test --stage 2 src/tools/rustfmt
-python3 "$X_PY" test --stage 2 src/tools/miri
+
+# Testing Miri is a bit more complicated.
+# We set the GC interval to the shortest possible value (0 would be off) to increase the chance
+# that bugs which only surface when the GC runs at a specific time are more likely to cause CI to fail.
+# This significantly increases the runtime of our test suite, or we'd do this in PR CI too.
+if [[ -z "${PR_CI_JOB:-}" ]]; then
+    MIRIFLAGS=-Zmiri-provenance-gc=1 python3 "$X_PY" test --stage 2 src/tools/miri
+else
+    python3 "$X_PY" test --stage 2 src/tools/miri
+fi
 # We natively run this script on x86_64-unknown-linux-gnu and x86_64-pc-windows-msvc.
 # Also cover some other targets via cross-testing, in particular all tier 1 targets.
 export BOOTSTRAP_SKIP_TARGET_SANITY=1 # we don't need `cc` for these targets

diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
@@ -165,7 +165,7 @@ target | std | notes
 `riscv64gc-unknown-none-elf` | * | Bare RISC-V (RV64IMAFDC ISA)
 `riscv64imac-unknown-none-elf` | * | Bare RISC-V (RV64IMAC ISA)
 `sparc64-unknown-linux-gnu` | ✓ | SPARC Linux (kernel 4.4, glibc 2.23)
-`sparcv9-sun-solaris` | ✓ | SPARC Solaris 10/11, illumos
+`sparcv9-sun-solaris` | ✓ | SPARC Solaris 11, illumos
 `thumbv6m-none-eabi` | * | Bare ARMv6-M
 `thumbv7em-none-eabi` | * | Bare ARMv7E-M
 `thumbv7em-none-eabihf` | * | Bare ARMV7E-M, hardfloat
@@ -184,7 +184,7 @@ target | std | notes
 `x86_64-fuchsia` | ✓ | Alias for `x86_64-unknown-fuchsia`
 [`x86_64-unknown-fuchsia`](platform-support/fuchsia.md) | ✓ | 64-bit x86 Fuchsia
 [`x86_64-linux-android`](platform-support/android.md) | ✓ | 64-bit x86 Android
-`x86_64-pc-solaris` | ✓ | 64-bit Solaris 10/11, illumos
+`x86_64-pc-solaris` | ✓ | 64-bit Solaris 11, illumos
 `x86_64-unknown-linux-gnux32` | ✓ | 64-bit Linux (x32 ABI) (kernel 4.15, glibc 2.27)
 [`x86_64-unknown-none`](platform-support/x86_64-unknown-none.md) | * | Freestanding/bare-metal x86_64, softfloat
 `x86_64-unknown-redox` | ✓ | Redox OS
@@ -342,7 +342,6 @@ target | std | host | notes
 [`x86_64-pc-nto-qnx710`](platform-support/nto-qnx.md) | ✓ |  | x86 64-bit QNX Neutrino 7.1 RTOS |
 [`x86_64-pc-windows-gnullvm`](platform-support/pc-windows-gnullvm.md) | ✓ | ✓ |
 `x86_64-pc-windows-msvc` | * |  | 64-bit Windows XP support
-`x86_64-sun-solaris` | ? |  | Deprecated target for 64-bit Solaris 10/11, illumos
 [`x86_64-unikraft-linux-musl`](platform-support/unikraft-linux-musl.md) | ✓ |   | 64-bit Unikraft with musl
 `x86_64-unknown-dragonfly` | ✓ | ✓ | 64-bit DragonFlyBSD
 `x86_64-unknown-haiku` | ✓ | ✓ | 64-bit Haiku

diff --git a/src/doc/rustdoc/src/read-documentation/search.md b/src/doc/rustdoc/src/read-documentation/search.md
@@ -72,6 +72,7 @@ the standard library and functions that are included in the results list:
 | [`stdout, [u8]`][stdoutu8] | `Stdout::write` |
 | [`any -> !`][] | `panic::panic_any` |
 | [`vec::intoiter<T> -> [T]`][iterasslice] | `IntoIter::as_slice` and `IntoIter::next_chunk` |
+| [`iterator<T>, fnmut -> T`][iterreduce] | `Iterator::reduce` and `Iterator::find` |
 
 [`usize -> vec`]: ../../std/vec/struct.Vec.html?search=usize%20-%3E%20vec&filter-crate=std
 [`vec, vec -> bool`]: ../../std/vec/struct.Vec.html?search=vec,%20vec%20-%3E%20bool&filter-crate=std
@@ -81,6 +82,7 @@ the standard library and functions that are included in the results list:
 [`any -> !`]: ../../std/vec/struct.Vec.html?search=any%20-%3E%20!&filter-crate=std
 [stdoutu8]: ../../std/vec/struct.Vec.html?search=stdout%2C%20[u8]&filter-crate=std
 [iterasslice]: ../../std/vec/struct.Vec.html?search=vec%3A%3Aintoiter<T>%20->%20[T]&filter-crate=std
+[iterreduce]: ../../std/index.html?search=iterator<T>%2C%20fnmut%20->%20T&filter-crate=std
 
 ### How type-based search works
 
@@ -95,16 +97,47 @@ After deciding which items are type parameters and which are actual types, it
 then searches by matching up the function parameters (written before the `->`)
 and the return types (written after the `->`). Type matching is order-agnostic,
 and allows items to be left out of the query, but items that are present in the
-query must be present in the function for it to match.
+query must be present in the function for it to match. The `self` parameter is
+treated the same as any other parameter, and `Self` is resolved to the
+underlying type's name.
 
 Function signature searches can query generics, wrapped in angle brackets, and
 traits will be normalized like types in the search engine if no type parameters
 match them. For example, a function with the signature
 `fn my_function<I: Iterator<Item=u32>>(input: I) -> usize`
 can be matched with the following queries:
 
-* `Iterator<u32> -> usize`
-* `Iterator -> usize`
+* `Iterator<Item=u32> -> usize`
+* `Iterator<u32> -> usize` (you can leave out the `Item=` part)
+* `Iterator -> usize` (you can leave out iterator's generic entirely)
+* `T -> usize` (you can match with a generic parameter)
+
+Each of the above queries is progressively looser, except the last one
+would not match `dyn Iterator`, since that's not a type parameter.
+
+If a bound has multiple associated types, specifying the name allows you to
+pick which one gets matched. If no name is specified, then the query will
+match of any of them. For example,
+
+```rust
+pub trait MyTrait {
+    type First;
+    type Second;
+}
+
+/// This function can be found using the following search queries:
+///
+///     MyTrait<First=u8, Second=u32> -> bool
+///     MyTrait<u32, First=u8> -> bool
+///     MyTrait<Second=u32> -> bool
+///     MyTrait<u32, u8> -> bool
+///
+/// The following queries, however, will *not* match it:
+///
+///     MyTrait<First=u32> -> bool
+///     MyTrait<u32, u32> -> bool
+pub fn my_fn(x: impl MyTrait<First=u8, Second=u32>) -> bool { true }
+```
 
 Generics and function parameters are order-agnostic, but sensitive to nesting
 and number of matches. For example, a function with the signature
@@ -134,6 +167,10 @@ Most of these limitations should be addressed in future version of Rustdoc.
     with that bound, it'll match, but `option<T> -> T where T: Default`
     cannot be precisely searched for (use `option<Default> -> Default`).
 
+  * Supertraits, type aliases, and Deref are all ignored. Search mostly
+    operates on type signatures *as written*, and not as they are
+    represented within the compiler.
+
   * Type parameters match type parameters, such that `Option<A>` matches
     `Option<T>`, but never match concrete types in function signatures.
     A trait named as if it were a type, such as `Option<Read>`, will match
@@ -183,7 +220,8 @@ slice = OPEN-SQUARE-BRACKET [ nonempty-arg-list ] CLOSE-SQUARE-BRACKET
 arg = [type-filter *WS COLON *WS] (path [generics] / slice / [!])
 type-sep = COMMA/WS *(COMMA/WS)
 nonempty-arg-list = *(type-sep) arg *(type-sep arg) *(type-sep)
-generics = OPEN-ANGLE-BRACKET [ nonempty-arg-list ] *(type-sep)
+generic-arg-list = *(type-sep) arg [ EQUAL arg ] *(type-sep arg [ EQUAL arg ]) *(type-sep)
+generics = OPEN-ANGLE-BRACKET [ generic-arg-list ] *(type-sep)
             CLOSE-ANGLE-BRACKET
 return-args = RETURN-ARROW *(type-sep) nonempty-arg-list
 
@@ -230,6 +268,7 @@ DOUBLE-COLON = "::"
 QUOTE = %x22
 COMMA = ","
 RETURN-ARROW = "->"
+EQUAL = "="
 
 ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
 DIGIT = %x30-39

diff --git a/src/doc/unstable-book/src/compiler-flags/check-cfg.md b/src/doc/unstable-book/src/compiler-flags/check-cfg.md
@@ -35,6 +35,9 @@ and `cfg!(name = "value")` call. It will check that the `"value"` specified is p
 list of expected values. If `"value"` is not in it, then `rustc` will report an `unexpected_cfgs`
 lint diagnostic. The default diagnostic level for this lint is `Warn`.
 
+The command line `--cfg` arguments are currently *NOT* checked but may very well be checked in
+the future.
+
 To enable checking of values, but to provide an empty set of expected values, use these forms:
 
 ```bash